Transparent conducting oxides are often deposited using magnetron sputtering due to scalability, reasonable cost, and quality. However, the desired oxide properties, including high conductivity and high transmittance, are usually less than optimum due to damage of the growing film caused by energetic particles. Examples of transparent conducting oxides include indium tin oxide (ITO) and aluminum doped zinc oxide (AZO).
The sputtering process necessarily makes use of positive ions accelerated towards the target to cause sputtering of the target material. A small fraction of ions “bounce” back as energetic atoms. The other energetic particles in the case of transparent conducting oxide sputtering are usually negative oxygen ions that are accelerated away from the target by the same field that accelerated positive ions toward the target. Both types of energetic particles, atoms and negative ions, can cause defects in the growing film, which may manifest themselves as scattering centers for electric carries, lowering their mobility, and as absorptions centers, lowering their transmittance. Therefore, in order to make a transparent conducting oxide, the bombardment with energetic particles needs to be eliminated or at least reduced.
The formation of energetic particles is inherently connected with the magnetron sputtering process. Therefore, one way of addressing the problem is to deal with energetic particles after their formation, but before they arrive at the substrate. In one approach, the process gas pressure is increased beyond the typical 1 to 10 millitorr in order to increase the likelihood of collisions between ions and atoms, thereby reducing the kinetic energy of the ions. This approach, however, will also cause collisions of the lower energy particles, especially sputtered atoms, which leads to a reduction in deposition rate and deterioration of film quality (e.g., voids in the film, reduced adhesion). In another approach, a blocking element or a shield (or shields) is placed over the racetrack of the magnetron target in order to block the harmful energetic ions that are mostly produced near the racetrack. This approach has the disadvantages that the blocking element gets coated and may produce particulates, and that the deposition rate is reduced. In still another approach, the substrate is placed off-axis such to avoid the impact of negative ions. As with the previous approach, the deposition rate is much reduced.
Plasma transport is well studied for filtered cathodic arc deposition. Electrons are magnetized by the magnetic field of a filter coil; the purpose of the filter to separate the cathodic arc plasma from the macroparticles that are also produced at cathode spots. In the filter, the electrons gyrate around the magnetic field lines and are thereby bound to the field. The center of the gyration motion is called the gyration center (of that motion). The gyration center is moving along the field lines unless a collision of the electron with an atom or an ion displaces it by about one gyration radius, after which the electron is bound to the neighboring magnetic field line. Since the field is curved in this specific configuration of a macroparticle filter, one can guide the electrons from the source to the substrate that is not in line-of-sight with the source.
As it is characteristic for plasmas, electron motion is coupled to positive ion motion by the Coulomb interaction. Thus, it is not possible to guide electrons away from the source without affecting the positive ions. As a result, the electrons and positive ions are moving together, where the transport mechanism is a combined magnetic (for electrons) and electric (for positive ions) mechanism. This is all well known for filters used in filtered cathodic arc deposition; a comprehensive review of theory and experiment was published in chapter 7 of the book “Cathodic Arcs” by A. Anders, New York: Springer, 2008.
The production of energetic negative ions in magnetron sputtering has been recognized, with the presence of such ions being detrimental to high quality film formation. Their impact causes displacements of lattice atoms in the coating and thereby the growth of crystalline grains is disturbed. The films are highly defective on an atomic level or from a crystallographic point of view. In essence, the electric field near the target, which is responsible for accelerating positive ions towards the target such as to cause sputtering from the target, is also responsible for accelerating negative ions (such as O−, O2−) away from the target and towards the substrate. The highest energy that a negative ion can obtain corresponds to the full applied target voltage (e.g., a 500 V target bias can create a 500 eV negative ion flying from the target to the substrate to be coated). This has been published, for example, in S. Mráz, and J. M. Schneider, “Energy distribution of O− ions during reactive magnetron sputtering,” Appl. Phys. Lett., vol. 89, no. 5, pp. 051502-3, 2006.